1. Field of the Invention
The present invention relates to a communication apparatus and a method and program for controlling this apparatus.
2. Description of the Related Art
A communication system composed of a base station (or access point, referred to as an “AP” below) and a plurality of communication terminals (or stations, referred to as “STAs” below) is known in the art. In such a communication system, each STA communicates with other STAs or connects to an external network via the AP.
Technology for preventing a decline in communication quality as caused by access contention or the like in such a communication system has been proposed. For example, EDCA (Enhanced Distributed Channel Access) and HCCA (HCF Controlled Channel Access) are available as control methods defined in IEEE 802.11e. EDCA is an access control method obtained by extending CSMA/CA. With EDCA, priority control is carried out in accordance with the order of priority of data. As a result, data having a high order of priority is transmitted in accordance with priority even in a case where access contention has occurred within the system.
HCCA is a control method that is an extension of a polling procedure. When communication starts, scheduling is carried out between an STA and AP in accordance with the type of data, taking the communication quality into consideration. Since scheduling is performed before the start of the data transmission in this control method, occurrence of access contention per se can be prevented.
Further, a control method of extending the communication band to the 40-MHz band is described in the draft of IEEE 802.11n. According to this control method, 20 MHz (the conventional communication band) is extended to 40 MHz, by way of example.
By way of example, a technique for controlling selection of a 20-MHz-only mode, 20/40-MHz mode and PCO mode described in the draft is known as a method of communication control that uses the technology mentioned above (Japanese Patent Laid-Open No. 2008-160758).
Also known is technology for giving notification of the frequency of occurrence of access contention. For example, according to a method defined in IEEE 802.11k, notification is given using information that has been set within a prescribed communication frame (Beacon frame body and Probe Response frame body). Specifically, “BSS Average Access Delay” information is used.
In the above-described communication system, assume that each STA is performing data communication with an external network or other STA continuously or periodically via an AP. Under these conditions, the problems set forth below arise if a further plurality of STAs issue connection requests to the AP so that the number of STAs using this AP increase.
If the sum total of communication bands requested by the STAs exceeds a prescribed amount in an EDCA communication period, there are cases where the communication band will be inadequate, even if priority control is carried out, and hence it will not be possible to assure the desired transmission rate.
Further, even in a case where the sum total of communication bands requested by the STAs is small, there are instances where data transmission waiting time due to access contention increases and the real-time nature of the data transmission cannot be assured when data transmission has been requested by a plurality of STAs of the same order of priority.
On the other hand, the HCCA communication period is such that access contention does not arise in this communication period because management and scheduling are performed by the AP. However, even if a new STA attempts to participate in communication during the HCCA communication period, this STA will not be able to communicate with the AP unless there is an availability in the communication band. Further, in an instance where an STA for which the order of priority of data communication is higher than that of an STA already in the connected state, a case where the STA having the higher order of priority is connected preferentially over the existing STA is conceivable.
In a known technique, if the sum total of communication bands requested by each of the STAs exceeds a prescribed amount, the communication band is extended in order to deal with data transmission requests issued by each of the STAs. However, if the communication band is extended (from the 20-MHz band to the 40-MHz band, for example) in the entire communication period, a problem may arise. For example, if there is another communication system nearby, there are instances where the radio channels interfere with each other.
In general, the frequency characteristic of a radio channel is not ideal. Consequently, a wireless terminal (STA) leaks power to an adjacent radio channel. In such cases there are instances where leakage power is detected as the carrier owing to application of the CSMA/CA scheme. In other words, if there is another communication system nearby, radio channels interfere with one another and the result is a decline in the communication band itself. If the frequency bands of two channels are used owing to extension of the communication band, this problem becomes even more pronounced and there are instances where radio channels equivalent to four channels are detected as the carrier owing to the inclusion of radio channels adjacent above and below.